Abstract(#br)Background(#br)validated and reliable tools measuring the level of competence about breastfeeding amongst health professionals who attend mothers in primary care are scarce.(#br)Objective(#br)to develop and validate a self-administered online questionnaire to assess basic competence about breastfeeding in the full range of health professionals attending lactating mothers in primary care.(#br)Design(#br)methodological study.(#br)Settings(#br)the study was conducted in 33 primary care centers administered by the Catalan Health Institute (Institut Català de la Salut) in the Girona Region (northeast Spain).(#br)Participants(#br)the questionnaire was sent to 398 health professionals who were likely to be in contact with breast feeding mothers. We included midwives, general... practitioners, pediatric nurses (who provide care for children from birth to 14 years old), non-pediatric nurses (who provide care for patients older than 14 year), pediatricians, and gynecologists. The random sampling was stratified by discipline to ensure representativeness.(#br)Methods(#br)the development and validation of the self-administered online questionnaire had five phases: (1) literature review, preparation of a draft for evaluation by an expert panel and pilot study; (2) design of a questionnaire based on observations from Phase 1; (3) questionnaire administration to a random sample of 398 health professionals, stratified by discipline; (4) re-testing after four weeks; and (5) assessment of construct validity using factor analysis and hypothesis-testing, comparing scores between professional groups and estimating effect size. Internal consistency was assessed by Cronbach alpha and reproducibility by the intraclass correlation coefficient.(#br)Findings(#br)The response rate was 69.3%. The final version of the questionnaire has 24 items. The uni-dimensionality of the questionnaire was confirmed by the factor analysis. The score differed significantly between professional groups ( F =12.904; p <0.001), among whom midwives achieved the highest score (129±10 points) and non-pediatric nurses the lowest (111±14 points).The effect sized values ranged from 0.6 and 1.5. Cronbach alpha was 0.870 (95% CI: 0.847, 0.891) and intraclass correlation coefficient was 0.856 (95%CI: 0.817, 0.888).(#br)Conclusions(#br)this is a valid and reliable questionnaire to assess a basic level of competence about breast feeding among primary care professionals involved in supporting breast-feeding mothers. This questionnaire may help to identify health professionals who require support to improve their capability to manage women who are breast feeding.